Glycoalkaloids are a group of compounds derived from plants and also referred to as steroidal alkaloids. The structure of glycoalkaloids contains an isoprenoid having a chain with 27 carbon atoms and a nitrogen atom, and it has been reported that 422 species of Solanum plants contain glycoalkaloids (Eich, Solanaceae and Convolvulaceae: Secondary Metabolite (2008), Springer, Chapter 7.8). As to a plant other than those belonging to the genus Solanum in the family Solanaceae, some plants belonging to the family Liliaceae are also known to contain glycoalkaloids. Among glycoalkaloids, important ones are chaconine and solanine from potatoes (Solanum tuberosum), and tomatine from tomatoes (Solanum lycopersicum), which belong to the genus Solanum in the family Solanaceae.
Potato is the fourth most produced crop in the world following corn, rice, and wheat. However, it is a well-known fact that toxic chaconine and solanine are contained in the bud coming out of the tuber or the aerial part of the plant. Symptoms of poisoning such as abdominal pain, dizziness, and mild disturbance of consciousness are caused by chaconine or solanine. Chaconine and solanine are easily accumulated in the tuber when the tuber is damaged or exposed to solar light, and thus there is a risk of poisoning accident caused by improper management of tuber.
Poisoning accidents caused by these frequently happen, and recently, a poisoning accident of glycoalkaloid occurred at an elementary school in Nara City, Japan on Jul. 16, 2009 (reported by Asahi.com). Potatoes are usually a safe food because they are managed such that the content of glycoalkaloid is maintained at 20 mg/100 g or less by storing the tuber of potato in a dark place etc. However, in consideration of the risk of such a poisoning accident described above, reducing glycoalkaloids in potatoes is a matter of concern to all of the persons who deal with potatoes such as the breeding, production, storage, transportation, sale, and purchase of potatoes, but has not been achieved to date. The reasons are as follows. A wild potato species with no glycoalkaloids has not been found, the biosynthetic pathway of glycoalkaloid is unconfirmed (Eich, Solanaceae and Convolvulaceae: Secondary Metabolite (2008), Springer, FIGS. 7.24A and 7.24B, and Ginzberg et al., Potato Research (2009) 52: 1-15), and the identification of gene involved in the biosynthetic pathway has not proceeded.
Glycoalkaloids exhibit toxicity such as cholinesterase inhibitory activity or membrane disruption effect, but in addition to this, it is known that glycoalkaloids exhibit medicinal effects such as an anti-cancer activity, a liver protective effect, an antispasmodic effect, an immune system promoting effect, an antifungal effect, an antiprotozoal effect, and a shellfish killing agent activity (Eich, Solanaceae and Convolvulaceae: Secondary Metabolite (2008), Springer). It has also been reported that esculeoside A, which is a metabolite of glycoalkaloids in tomato, exhibits various physiological effects (Nohara et al., J. Nat. Prod. (2010) 73: 1734-1741). However, research and development on suppressing the metabolites or efficient production thereof have hardly proceeded since the biosynthetic pathway thereof is not known.
Several enzyme genes catalyzing the transglycosylation process following the aglycone biosynthesis process have been reported (McCue et al., Plant Sci. (2005) 168: 267-273; McCue et al., Phytochemistry (2006) 67: 1590-1597; McCue et al., Phytochemistry (2007) 68: 327-334). However, in McCue et al., Plant Sci. (2005) 168: 267-273, the gene of UDP-galactosyltransferase, which mediates the conversion of solanidine, which is aglycone, to γ solanine, and a strain in which the gene is suppressed have been reported, but the production of chaconine has not been suppressed at all (McCue et al., Plant Sci. (2005) 168: 267-273, FIG. 2). In McCue et al., Plant Sci. (2005) 168: 267-273, the gene of UDP-glucosyltransferase, which mediates the conversion of solanidine to 7 chaconine, and a strain in which the gene is suppressed have been reported, but the production of either of chaconine and solanine is hardly suppressed (McCue et al., Phytochemistry (2006) 67: 1590-1597, FIG. 5). In McCue et al., Phytochemistry (2007) 68: 327-334, the gene of rhamnosyl transferase, which mediates the conversion of β chaconine to α chaconine and β solanine to α solanine, has been reported, but the β-form and γ-form are increased by the suppression of the gene, although the α-form is decreased. As seen from these, by the suppression of the transglycosylation process, the molecular species of glycoalkaloids can be changed but it is very difficult to control the total amount of glycoalkaloids. Recently, an enzyme gene, which catalyzes the oxidative pathway involved in the biosynthetic pathway of glycoalkaloids, has been reported (WO 2011/025011). However, the specific enzyme reaction has remained unclear.
There is a report of an attempt to decrease in glycoalkaloids by overexpressing biosynthetic genes of plant sterols or plant hormones (Arnqvist et al., Plant Physiol. (2003) 131: 1792-1799). However, the amount of glycoalkaloids can only be reduced to about half at most, and thus an effective means has not been provided in modifying the pathway (Arnqvist et al., Plant Physiol. (2003) 131: 1792-1799, FIG. 5).
Glycoalkaloids is characterized by containing a nitrogen molecule. Research on this biosynthesis is poor. In Heftmann, Phytochemistry (1983) 22: 1843-1860, a review, it is reported that the hydroxyl group at the terminal position 26 is simply replaced with an amino group, and the donor thereof is glycine or alanine in potato of Solanaceae.